I want to create a "boss and minions" encounter for my low-to-mid-level party, and I love the idea of legendary and lair actions as a way to give the encounter an appropriately "epic" feel. Problem is, I can't seem to find any material in the DMG that directly addresses how these abilities affect the CR of the creature in question.
The DMG (p. 278) has this to say:
When calculating a monster's damage output, also account for special off-turn damage-dealing features, such as auras, reactions, legendary actions, or lair actions.
Does this mean that I perform the "average damage across three rounds" calculation, assuming that legendary actions get used as frequently as possible on a party of four adventurers, to help determine the Offensive CR of the creature? What about if I have a not-specifically-damaging feature (such as a creature that attempts to charm another creature as a legendary action)?
Are there official rules to handle situations like these, or does this fall into "estimate then playtest" territory?